Method of degassing a cathode-ray tube prior to sealing

ABSTRACT

A method for making a cathode-ray tube including baking the envelope and mount assembly and simultaneously exhausting gases from the envelope, and then sealing (tipping off) the exhausted envelope. During the baking and exhausting of the tube, the following steps are carried out first sequentially and then simultaneously: (a) applying radio-frequency energy to the mount assembly and (b) passing electric current through the cathode heater.

United States Patent 11 1 Sawicki 1 Nov. 25, 1975 METHOD OF DEGASSING ACATl-lODE-RAY TL'BE PRIOR TO SEALING Primary E.\'u111111erRoy LakeAssistant E.\uminer.lames W. Davie 75 I 1 Imamor gz Smnle Sawlckl'Scranton Attorney, Age/1L or FirmG. H. Bruestle; L.

I Greenspan {73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Mar. 25, I974 [57] ABSTRACT [31] A L N 454 153 A method formaking a cathode-ray tube including baking the envelope and mountassembly and simultaneously exhausting gases from the envelope. and then[52] S f, 6/19; 316/30 sealing (tipping off) the exhausted emelope.During i511 9/18 the baking and exhausting of the tube. the following 8]field Search 316/]71 steps are carried out first sequentially and thensimul 3l6/30- 13 53/88 taneously: (a) applying radio-frequency energy tothe mount assembly and (b) passing electric current [56] References andthrough the cathode heater.

UNITED STATES PATENTS 4 CM 1 D F 2.532.315 12/1950 Johnson ct al 316/30rawmg 'gure l2lll098765432lllllll l3 A C l4 l5 l6 l7 l8 l9 20 El 22' 2324 25 26 27 28 P08. lO-I8 2 P0335-42 POS.

POS.24-28 METHOD OF DEGASSING A CATI-IODE-RAY TUBE PRIOR TO SEALINGBACKGROUND OF THE INVENTION This invention relates to a noveLmethod fordegassing a cathode-ray tube, particularly the mount-assembly structuresthereof, during the simultaneous exhausting and baking steps for makingthe tube. prior to sealing the tube.

In fabricating a cathode-ray tube. a luminescent screen and variousconductive coatings are applied to various internal surfaces of theenvelope which includes a neck portion. A mount assembly, supported on aglass stem and including the electron gun or guns, is sealed into theneck portion of the tube. The tube, which is open to the atmospherethrough a glass tubulation connected to the stem. is baked at about 300to 450C and is simultaneously exhausted to a relatively low pressure.Then, the tube is tipped off; that is, the tubulation is sealed.Apparatus for carrying out this process is described in the prior art;for example, U.S. Pat. No. 2,532,3l5 to M. E. Johnson et al and U.S.Pat. No. 3,115,732 to I. F. Stewart.

Near the end of the baking and exhausting cycle and prior to tippingoff, radio-frequency energy at about l.0 to l.5 magahertz is applied tothe mount assembly to degas the mount structures by heating. Then,electric current is passed through the cathode heater to heat thecathode to decompose the cathode coating material and to degas thecathode structure. It has been suggested to perform these latter twosteps either sequentially or simultaneously. Each of the sequential andsimultaneous orders of processing has disadvantages. Sequentialapplication of radio-frequency energy and heater current does notproduce tubes having the highest cathode emission characteristics.Simultaneous application of radio-frequency energy and heater currentfrequently results in an increase in the proportion oftubes that isrejected for unsatisfactory performance.

SUMMARY OF THE INVENTION In the novel method, radio-frequency energy andheater current are first applied sequentially and then are appliedsimultaneously to the mount assembly and cathode heater respectivelyduring the simultaneous baking and exhausting steps prior to sealing.The initial sequential processing performs the previous functions ofdegassing the structures and decomposing the cathode coating. Thesubsequent simultaneous processing raises the temperatures of all thestructures above 450C to again outgas them but in a manner that preventsredeposition of materials released from other structures. As a result,the finished tubes exhibit higher emission currents from the cathodesand there is a high yield of satisfactory tubes.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a schematic planview of an apparatus for practicing the novel method showing therelative locations of the heating zones and the process ing positions.

DETAILED DESCRIPTION OF THE INVENTION The invention may be practiced inthe continuous apparatus disclosed in U.S. Pat. No. 2,532,315 to M. E.Johnson et al. The apparatus shown in the sole FIG- URE is similar tothe apparatus described in the Johnson et al patent; however, theapparatus employs the improved cart disclosed in U.S. Pat. No. 3.115,732 to J. F. Stewart. The apparatus shown in the sole FIGUREcomprises a train of carts A moving counter clockwise along a closed,elongated loop. A tunnel oven B of a generally U-shaped plan is locatedover a portion of the train of carts in a manner to enclose the tubesbeing processed. The tunnel is divided into 28 zones that are numbered Ithrough 28. Except for zones 13 and 14, the zones are straight and ofequal length of about 10 feet. Bus bars delivering electrical energy forprocessing the tubes are located along zones 15 through 24 and aredivided into processing positions C numbered 1 through 42. Theprocessing positions are each about 28 inches in length.

A sequence of 25 V-size color television picture tubes (assembled butnot yet exhausted and sealed) is loaded successively, one tube to acart, onto the train of carts A passing continuously into and throughthe tunnel oven B of this apparatus at the rate of about 54 to carts perhour. Each tube includes an envelope having a faceplate, a funnelportion, and a glass-neck portion. The neck portion is closed at one endby a glass stem having metal stem leads and a glass tubulation extendingoutwardly therefrom. The metal stem leads also extend inwardly andsupport the mount assembly of the tube. The mount assembly includes theelectron guns of the tube. Each gun includes an indirectly heatedthermionic cathode having a heated side and an emitting side, a cathodeheater spaced from the heated side of the cathode, and a grid (GI)spaced from the emitting side of the cathode. The tubulation isconnected to equipment for exhausting the tube of gas, and the stemleads are connected so that a current may be passed through the cathodeheater. A coil is disposed about the neck so that radio-frequency energymay be applied to the mount assembly of the tube.

Each cart passes through the tunnel oven B in about two to three hours.The temperatures of the zones increase from about room temperature (atzone 1) to about 440C (at zone 11) and then decrease to about C (at zone28). During most of the transit through the oven B (zones 1 to 25),pumps are removing gas from the interior of each tube through theexhaust tubulation extending from the stem that is sealed into the tubeneck. Starting at zone 17 (processing positions 10 through 18),radio-frequency energy at about 1.0 to L5 megahertz is applied to themount assembly according to the schedule in the TABLE. No radiofrequencyenergy is applied at processing positions 19 through 23. However, acathode-heater current of about 1.] amp is passed through the cathodeheaters with an applied voltage of about 12.0 volts dc at position 23.The applied voltage is higher than the normal operating voltage of about6.3 volts, and causes the cathode temperature to rise about to 250 Cabove the normal operating temperatureof about 800C. At processingpositions 24 through 28, both radiofrequency energy and cathode-heatercurrent are applied simultaneously to the mount assembly and the cathodeheaters respectively according to the schedule in the TABLE. The heatingeffect on the cathode, on grid No. 1 (GI) and grid No. 3 (G3) of thetube at the various positions is also shown in the TABLE. At positions29 to 34, heat is applied to melt a portion of the tubulation and toseal (tip-off) the tube interior from the atmosphere.

The prior factory process follows the schedule shown in the TABLE exceptthat the radio-frequency energy (rf) and the cathode heater current (If)are both zero at positions 24 through 28. This permits the temperatureof the cathode, ofGl, and of G3, to drop continuously to about 200C atposition 28 (in zone 2| Contrary to this, in the novel method, thesimultaneous application of radio-frequency energy and cathode-heatercurrent at positions 24 through 28 raises the temperature of the cathodeand the grid No. 1 above 450C, and preferably above 600C. Thisadditional high-temperature heating is believed to clean and degas thesurfaces of the various structures without causing lifting of thecoating on the cathode. Results have shown an increase incathode-emission current in the finished tubes of about 8 percent, and areduction of reject tubes due to lifted cathode coatings and othercauses, as compared with the prior process which employs only thesequential application of radio-frequency energy and cathodeheatercurrent.

While the above example employs a particular schedule on a particularcathode-ray-tube type and on a particular machine, the novel method isnot limited to any of these. The method may be practiced with differentschedules and various tubes and on a variety of machines. For example,the invention may be practiced on a stationary machine with periodictube processing. However, the novel method does require first thesequential application of radio-frequency energy and cathode-heatercurrent, followed by the simultaneous application of bothradio-frequency energy and cathode-heater current to the mount assemblyand cathode heater respectively, and preferably is conducted for atleast 2 minutes.

TABLE Position rt ll TC TC TC Amps Amps Cathode GI G3 7.0 0 200 300 250l l 7.0 0 260 400 325 12 7.0 0 320 450 375 13 7.0 0 380 475 425 M 7.0 0420 500 475 15 9.0 0 420 575 505 to 9.0 0 480 575 575 I7 l2.5 0 560 750625 I8 l2.5 0 640 770 675 I9 0 0 560 550 600 0 0 480 500 525 21 0 0 400450 450 22 0 0 320 400 375 23 0 1.1 950 350 300 24 9.0 0.8 850 525 44025 9.0 0.8 860 $70 480 26 9.0 0.8 870 600 520 27 9.0 0.8 880 620 560 289.0 0.8 890 640 560 29 0 0 400 480 420 30 0 0 320 380 350 I claim:

1. In a method for making a cathode-ray tuhe comprising an envelope anda mount assembly including at least one cathode and a heater thereforsealed in said envelope, said method including the steps of baking saidenvelope and mount assembly at about 300 to 450C and simultaneouslyexhausting gases from said envelope and then sealing said envelope,

the steps during said baking and exhausting of said envelope comprisingfirst l sequentially and then (2) simultaneously (a) applyingradio-frequency energy to said mount assembly and (b) passing electriccurrent through said heater, whereby said simultaneous application step(2) heats said mount assembly including said cathode above 450C.

2. The method defined in claim I wherein said simultaneous step (2) isconducted for at least 2 minutes.

3. In a method for making a cathode-ray tube comprising an envelopehaving a neck portion, a mount assembly sealed in said neck portion,said mount assembly including at least one indirectly-heated thermioniccathode having an emitting side and a heated side. a grid spaced fromsaid emitting side of said cathode and a heater spaced from the heatedside of said cathode, and an exhaust tubulation extending from saidenvelope,

said method including the steps of baking said envelope and mountassembly at about 300 to 450C. simultaneously exhausting gases frominside said envelope through said exhaust tubulation and, in sequenceduring said baking and exhausting steps:

a. applying radio-frequency energy to said mount assembly to heat themetal parts thereof above 450C.

b. removing said radio-frequency energy,

c. passing electric current through the heater of said mount assembly toheat the cathode thereof above 450C then simultaneously passing electriccurrent through said heater and applying radio-frequency energy to saidmount assembly to heat both said cathode and the metal parts of saidmount assembly above 450C,

e. removing said simultaneously applied electric current from saidheater and radio-frequency energy from said mount assembly, and

f. sealing off said exhaust tubulation envelope to seal the interior ofsaid tube from the ambient.

4. The method of claim 3 wherein said simultaneous application step (d)heats said grid and said cathode above 600C.

1. In a method for making a cathode-ray tube comprising an envelope anda mount assembly including at least one cathode and a heater thereforsealed in said envelope, said method including the steps of baking saidenvelope and mount assembly at about 300* to 450*C and simultaneouslyexhausting gases from said envelope and then sealing said envelope, thesteps during said baking and exhausting of said envelope comprisingfirst (1) sequentially and then (2) simultaneously (a) applyingradio-frequency energy to said mount assembly and (b) passing electriccurrent through said heater, whereby said simultaneous application step(2) heats said mount assembly including said cathode above 450*C.
 2. Themethod defined in claim 1 wherein said simultaneous step (2) isconducted for at least 2 minutes.
 3. In a method for making acathode-ray tube comprising an envelope having a neck portion, a mountassembly sealed in said neck portion, said mount assembly including atleast one indirectly-heated thermionic cathode having an emitting sideand a heated side, a grid spaced from said emitting side of said cathodeand a heater spaced from the heated side of said cathode, and an exhausttubulation extending from said envelope, said method including the stepsof baking said envelope and mount assembly at about 300* to 450*C,simultaneously exhausting gases from inside said envelope through saidexhaust tubulation and, in sequence during said baking and exhaustingsteps: a. applying radio-frequency energy to said mount assembly to heatthe metal parts thereof above 450*C, b. reMoving said radio-frequencyenergy, c. passing electric current through the heater of said mountassembly to heat the cathode thereof above 450*C, d. then simultaneouslypassing electric current through said heater and applyingradio-frequency energy to said mount assembly to heat both said cathodeand the metal parts of said mount assembly above 450*C, e. removing saidsimultaneously applied electric current from said heater andradio-frequency energy from said mount assembly, and f. sealing off saidexhaust tubulation envelope to seal the interior of said tube from theambient.
 4. The method of claim 3 wherein said simultaneous applicationstep (d) heats said grid and said cathode above 600*C.